----->>>Hi: Are you baptized? If so, will your grandparents turn you in to the elders? This could leave you out in the cold in more ways then one. If you do want to go ahead and tell them how you feel, keep to one of two topics and avoid getting confentational. <<<-------
One other thing to bear in mind, depending on the status of your grandfather, whether he himself is an elder, serves on the body, etc, there may be an effort made to meet with you to 'encourage' you to rethink your choices. During this meeting, while their goal is to 'correct' your thinking, they will be collecting 'evidence' of your degree of 'standing off, away from' which may be used if they find and agree that your presence in the congregation may pose a threat. First and foremost, they will want to meet with you to dissuade you from departing. Failing that effort however their next line of reasoning is to build a case for cutting you off from the congregation, even though you are not baptised! We've all heard those wonderful annoucements...'so and so is no longer a publisher' which is 'truthspeak' for 'they are not baptised so we cannot disfellowship but they have engaged in activities that were they baptised they would be removed so treat them as though they have'. That's the time when the teen in the audience whose name has just been spoken over the p.a. system wishes they would have simply 'marked' them instead of blown them out of the circle of 'friends'.
I hope things go well with you and that the wonderful and warm suggestions posted here from others have provided you with some food for thought. In all that you do...remember, God and the Organization are not synonymous...you can love ONE without allegience/affliation to/with the other.